1. Field of the Invention
The present invention relates to an alkaline storage battery comprising a positive electrode, a negative electrode utilizing a hydrogen-absorbing alloy, and an alkaline electrolyte and the negative electrodes used in such an alkaline storage battery. More particularly, a feature of the invention is to improve the above-described negative electrode for alkaline storage battery for the purpose of restricting oxidation of the hydrogen-absorbing alloy by the alkaline electrolyte in the case of repeated charging and discharging, so that an alkaline storage battery with excellent cycle life can be obtained.
2. Description of the Related Art
Conventionally, nickel-cadmium storage batteries have been commonly used as alkaline storage batteries. In recent years, nickel-metal hydride storage batteries using hydrogen-absorbing alloys as their negative electrodes have drawn considerable attention from the viewpoints that they have higher capacity than nickel-cadmium storage batteries and are more environmentally safe because of being free of cadmium.
As the alkaline storage batteries of nickel-metal hydride storage batteries have been used in various portable devices and hybrid electric cars, demands for further higher performance in the nickel-metal hydride storage batteries have been increasing.
In such alkaline storage batteries, hydrogen-absorbing alloys such as a LaNi5 type hydrogen-absorbing alloy of rare earth-nickel intermetallic compound having a CaCu5 crystal structure as its main phase and a hydrogen-absorbing alloy containing Ti, Zr, V and Ni as its component elements and having Laves phase as its main phase, have been generally used for their negative electrodes.
However, these hydrogen-absorbing alloys generally do not necessarily have sufficient hydrogen-absorbing capability, and it has been difficult to increase the capacity of the alkaline storage batteries further.
In recent years, in order to improve the hydrogen-absorbing capability of the rare earth-nickel hydrogen-absorbing alloy, it has been proposed to use a rare earth-Mg—Ni-based hydrogen-absorbing alloy having a Ce2Ni7 type, a CeNi3 type, or the similar type crystal structure, rather than a CaCu5 type, by adding Mg or the like to the rare earth-nickel hydrogen-absorbing alloy. (See, for example, Japanese Published Unexamined Patent Application No. 2002-69554.)
Such a rare earth-Mg—Ni-based hydrogen-absorbing alloy is easily cracked in general and a new phase having a high reactivity is easily formed which contributes to discharge reaction, and therefore, high rate discharge performances as well as discharge performances under low temperature environments are relatively favorable.
However, corrosion resistance in the above-described rare earth-Mg—Ni-based hydrogen-absorbing alloy is degraded, and the above-described hydrogen absorbing alloy itself is deteriorated because of oxidation by an alkaline electrolyte during repeated charging and discharging. Also, the alkaline electrolyte in an alkaline storage battery is gradually consumed, and therefore, the amount of alkaline electrolyte contained in a separator is decreased. As a result, inner resistance in the alkaline storage battery is increased and cycle life thereof is greatly decreased.
In this connection, there has been proposed a storage battery, such as disclosed in JP-A 7-235304, which contains a nonionic surface active agent as an inhibitor of element-elution from a hydrogen-absorbing alloy, that is to say, an inhibitor of oxidization of the hydrogen-absorbing alloy, so that the storage battery's cycle life is improved.
However, even if such a nonionic surface active agent is contained in the storage battery, elution of the elements, namely, oxidization of the hydrogen-absorbing alloy is not fully inhibited, and improvement of the alkaline storage battery's cycle life is not fully attained.
Also, there has been proposed a storage battery, such as disclosed in JP-A 2005-190863 and JP-A 4-284354, which comprises a negative electrode wherein a fluorine resin is added to improve the storage battery's cycle life.
However, if the fluorine resin is added to the negative electrode, while the cycle life is improved, discharge performances are degraded.